Method of removing noise from image signal

ABSTRACT

In order to provide a noise removal method for removing noise signals mixed into an image signal without deteriorating picture quality of the overall image, such zigzag noise signals that the difference between the output levels of two pixels adjacent to each other along a noise generation direction alternately takes positive and negative vales at least three times are detected from noise signals mixed into an image signal. Then, a specific pixel is noted among a plurality of pixels corresponding to the noise signals, for calculating a mean value of the output levels of the noise signals with reference to the specific pixel and correcting the output level of a noise signal corresponding to the said specific pixel with the said mean value.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a noise removal method fordetecting and removing noise appearing in an image signal picked up withan image pickup sensor such as a digital camera.

[0003] 2. Description of the Background Art

[0004]FIG. 10 is a schematic block diagram of a general digital stillcamera 100. In the digital still camera 100, an analog image signalpicked up with an image pickup sensor 105 such as a CCD (charge coupleddevice) sensor or a CMOS sensor is converted to a digital signal andthereafter subjected to various image processing such as pixelinterpolation, color space conversion and edge enhancement in an imageprocessing part 106, as shown in FIG. 10. The Image data subjected tosuch image processing is culled and finder-displayed on a liquid crystalmonitor 109 or the like, compression-coded in the JPEG (jointphotographic experts group) system or the like and stored in a memorycard 110 such as a nonvolatile memory, or output to an external devicesuch as a personal computer through an interface 111. Referring to FIG.10, numeral 101 denotes an optical lens, numeral 102 denotes a colorcorrection filter, numeral 103 denotes an optical LPF (low-pass filter),numeral 104 denotes a color filter array, and numeral 107 denotes adriving part driving/controlling the image sensor 105 and the like.

[0005] However, a noise signal is disadvantageously mixed into the imagesignal picked up with the aforementioned image pickup sensor 105, todeteriorate the picture quality. When the image signal mixed with thenoise signal is subjected to edge enhancement, the noise signal is alsoenhanced to further deteriorate the picture quality. This type of noisesignal results from the hardware structure of an A/D converter oremployment of pixel values around a noted specific pixel in pixelinterpolation, for example. It is difficult to individually remove thistype of noise signal, and hence the overall image signal is generallypassed through the LPF 103 for blurring the noise signal appearing inthe image signal. In this case, however, the edge-enhanced part is alsoblurred to disadvantageously reduce the overall picture quality.

SUMMARY OF THE INVENTION

[0006] According to a first aspect of the present invention, a method ofremoving noise from an image signal comprises steps of (a) detectingsuch a zigzag signal that the difference between the output levels oftwo adjacent pixels alternately takes positive and negative values atleast three times in pixel arrangement along a prescribed direction and(b) noting a specific pixel among a plurality of pixels corresponding tothe zigzag signal and calculating a mean value of the output levels ofsignals corresponding to the plurality of pixels with reference to thespecific pixel for correcting the output level of a signal correspondingto the specific pixel to said mean value.

[0007] The method according to the first aspect can extract a noisesignal having a zigzag output level appearing around a Nyquist frequencyand individually correct the same. Therefore, the zigzag noise signalcan be removed without deteriorating the overall picture quality.

[0008] According to a second aspect of the present invention, the zigzagsignal is generated as a straight line in the image signal in the step(a), and the method calculates a weighted mean of the output level ofthe zigzag signal with reference to the specific pixel for correctingthe output level of the signal corresponding to the specific pixel tothe weighted mean in the step (b).

[0009] The method according to the second aspect can remove a generatednoise signal as a straight line appearing around the Nyquist frequencyfrom the image signal in higher accuracy.

[0010] According to a third aspect of the present invention, the imagesignal is a two-dimensional image signal formed by horizontal andvertical lines, and the zigzag signal is intersectionally generated intwo directions of the horizontal and vertical lines in the image signalin the step (a), while the method calculates a weighted mean of theoutput level of the zigzag signal generated along the two directionswith reference to the specific pixel at the intersection of thehorizontal and vertical lines for correcting the output level of thesignal corresponding to the specific pixel to the weighted mean in thestep (b).

[0011] The method according to the third aspect can detect a noisesignal at the intersection of two zigzag noise signals appearing in twodirections of vertical and horizontal lines from a two-dimensional imagesignal and remove the same in high accuracy.

[0012] Accordingly, an object of the present invention is to provide anoise removal method for individually removing a noise signal mixed intoan image signal without lowering picture quality.

[0013] The foregoing and other objects, features, aspects and advantagesof the present invention will become more apparent from the followingdetailed description of the present invention when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIGS. 1(a) shows signal levels corresponding to respective pixelsof a line sensor and FIG. 1(b) schematically shows pixel arrangement ofthe line sensor for illustrating a noise removal method according to anembodiment 1 of the present invention;

[0015]FIG. 2 is a flow chart showing a procedure of detecting andremoving noise signal levels shown in FIG. 1(a);

[0016]FIG. 3(a) illustrates signal levels corresponding to respectivepixels of a line sensor, and FIG. 3(b) schematically shows pixelarrangement of the line sensor;

[0017] FIGS. 4(a) shows signal levels corresponding to respective pixelsof a line sensor and FIG. 4(b) schematically shows pixel arrangement ofthe line sensor for illustrating a noise removal method according to anembodiment 2 of the present invention;

[0018] FIGS. 5(a) shows signal levels corresponding to respective pixelsof an image pickup sensor and FIG. 5(b) schematically shows pixelarrangement of the image pickup sensor for illustrating the noiseremoval method according to the embodiment 2 of the present invention;

[0019]FIG. 6 is a flow chart showing an exemplary procedure of removingzigzag noise signals illustrated in FIGS. 4(a) and 5(a);

[0020]FIG. 7(a) shows signal levels corresponding to respective pixelsof a vertical line of an image pickup sensor and FIG. 7(b) shows signallevels corresponding to respective pixels of a horizontal line of theimage pickup sensor for illustrating a noise removal method according toan embodiment 3 of the present invention;

[0021]FIG. 8 schematically shows pixel arrangement of the image pickupsensor;

[0022]FIG. 9 is a flow chart showing exemplary noise removal processingaccording to the embodiment 3; and

[0023]FIG. 10 is a schematic block diagram of a general digital stillcamera.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Embodiment 1.

[0025] FIGS. 1(a) and 1(b) are schematic diagrams for illustrating anoise removal method according to an embodiment 1 of the presentinvention. FIG. 1(a) illustrates the output levels (hereinafter referredto as signal levels) of signals corresponding to respective pixels of aline sensor such as a CCD sensor or a CMOS sensor, and FIG. 1(b)schematically shows pixel arrangement L1 of the line sensor. The linesensor having the one-dimensional pixel arrangement L1 shown in FIG.1(b) is formed by linearly arranging a plurality of photodiodes, andsymbols n[−2], n[−1], n[0], n[1], n[2], . . . denote pixelscorresponding to the photodiodes respectively.

[0026] As shown in FIG. 1(a), noise signal levels I[−2], I[−1], I[0],I[1], I[2], . . . are detected in correspondence to the pixels n[−2],n[−1], n[0], n[1], n[2], . . . respectively. These types of noise signallevels are detected when having zigzag values. The output levels ofthese noise signals are detected when the difference between the signallevels of two pixels adjacent to each other in a noise generationdirection repetitively changes while alternately taking positive andnegative values at least three times such that the difference ispositive, negative, positive, negative, . . . , specifically speakingwhen satisfying at least either one of the following conditionalexpressions (A-1) and (A-2):

I[−2]<I[−1], and I[−1]>I[0], and I[0]<I[1]  (A-1)

I[2]<I[1], and I[1]>I[0], and I[0]<I[−1]  (A-2)

[0027] A noise signal satisfying either the aforementioned conditionalexpression (A-1) or (A-2) is determined as a signal around a Nyquistfrequency, and not regarded as an original image signal. The term“Nyquist frequency” stands for the inverse number of the maximumseparation in time (Nyquist interval) which can be given to regularlyspaced instantaneous samples of a wave of specified bandwidth forcomplete determination of the waveform of signal. This type of noisesignal readily appears around the Nyquist frequency, to be mixed intothe original signal due to the hardware structure of an A/D converter orin pixel interpolation. When a color filter array of a primary colorsystem of “R (red component)”, “G (green component)”and “B (bluecomponent)” is arranged in correspondence to respective pixels in aphotosensitive part of an image pickup sensor, for example, every pixelof an image signal output from the image pickup sensor is subjected topixel interpolation for interpolating the remaining two color componentswith the pixel value of the color component around the pixel. As to apixel having original data of an “R” component, for example, “B”components of four pixels obliquely adjacent to this pixel arearithmetically averaged thereby calculating pixel interpolation data ofthe remaining color components not provided in this pixel. It has beenproven that difference in signal level is readily caused between thepixel interpolation data and the original data to readily result in anoise signal having the aforementioned zigzag value.

[0028]FIG. 2 is a flow chart showing an exemplary procedure of detectingand removing the output level of such a zigzag noise signal. As shown inFIG. 2, an image signal picked up by the line sensor L1 is A/D convertedand thereafter input in an image processing part (not shown) (ST1), andsubjected to image processing such as pixel interpolation, color spaceconversion and edge enhancement (ST2).

[0029] Then, the process shifts to a step ST3 for examining the signallevel processed in the aforementioned steps ST1 and ST2 and determiningwhether or not a noise signal satisfying the aforementioned conditionalexpression (A-1) or (A-2) is mixed into the same (ST4). When detectingthe noise signal satisfying the aforementioned conditional expression(A-1) or (A-2), the process shifts to a step ST5.

[0030] When detecting no noise signal satisfying the aforementionedconditional expression (A-1) or (A-2), on the other hand, the processshifts to a step ST6 for outputting an uncorrected image signal andending the noise removal processing at the step ST6.

[0031] At the step ST5, a signal level I[0] corresponding to a specificpixel n[0] to be corrected is corrected to: $\begin{matrix}{\left. {I\lbrack 0\rbrack}\leftarrow{\sum\limits_{i = 1}^{m}\quad \frac{{2{I\lbrack 0\rbrack}} + {I\lbrack i\rbrack} + {I\left\lbrack {- i} \right\rbrack}}{4m}} \right.\left( {m:{{natural}\quad {number}}} \right)} & ({F1})\end{matrix}$

[0032] Thus, a noise signal can be removed from the specific pixel. Theabove formula (F1) expresses an arithmetic mean value of 4 m noisesignal levels. When m=1, the signal level I[0] can be corrected to avalue (2×I[0]+I[1]+I[−1])/4. In the above formula (F1), it isparticularly preferable that the value m is a two's exponentiation(2^(n): n represents an integer of at least 0). This is because thestructure of carrying out division by the two's exponentiation (2^(n+2))is ordinarily implemented by a shift arithmetic unit in hardware, and adivider having a large gate scale is required to complicate the circuitstructure when performing division by a numerical value other than thetwo's exponentiation. After the corrected image signal is output at thesubsequent step ST6, the noise removal processing according to theembodiment 1 is ended.

[0033] While the noise signal is detected and corrected at theaforementioned steps ST3 to ST5 after serial image processing such asimage interpolation, color space conversion and edge enhancement isexecuted at the aforementioned step ST2 in the above noise removalprocessing, the embodiment 1 of the present invention is not restrictedto this but the aforementioned steps ST3 to ST5 may be assembled intothe serial image processing in the aforementioned step ST2. For example,color space conversion and edge enhancement may be executed afterexecuting image interpolation in the image processing at theaforementioned step ST2 and thereafter detecting and correcting thenoise signal at the aforementioned steps ST3 to ST5.

[0034] Thus, the embodiment 1 can detect the noise signal having anoutput level changing in a zigzag manner and individually remove thesame, whereby a noise signal resulting from the hardware structure of anA/D converter or particularly a noise signal resulting from pixelinterpolation can be removed without reducing the overall picturequality.

[0035] Modification of Embodiment 1.

[0036] Noise signals shown in FIG. 3(a) may alternatively be detected inplace of the noise signals shown in FIG. 1(a). FIG. 3(a) illustratessignal levels corresponding to respective pixels of a line sensor suchas a CCD sensor or a CMOS sensor, and FIG. 3(b) schematically showspixel arrangement L2 of the line sensor. Similarly to the aforementionedline sensor, the line sensor shown in FIG. 3(b) is formed by linearlyarranging a plurality of photodiodes, and symbols n[−2], n[−1], n[0],n[1], n[2], . . . denote pixels corresponding to the photodiodesrespectively. Further, zigzag noise signal levels I[−2], I[−1], I[0],I[1], I[2], . . . are detected in correspondence to the pixels n[−2],n[−1], n[0], n[1], n[2], . . . respectively, as shown in FIG. 3(a). Inother words, noise signals satisfying at least either one of thefollowing conditional expressions (B-1) and (B-2) are detected:

I[−2]>I[−1], and I[−1]<I[0], and I[0]>I[1]  (B-1)

I[2]>I[1], and I[1]<I[0], and I[0]>I[−1]  (B-2)

[0037] Noise removal processing according to this modification may beexecuted by employing the above conditional expressions (B-1) and (B-2)in place of the aforementioned conditional expressions (A-1) and (A-2)in the aforementioned embodiment 1. Thus, an effect similar to that ofthe aforementioned embodiment 1 can be attained.

[0038] Embodiment 2.

[0039] The noise removal method according to the aforementionedembodiment 1 may alternatively be applied to an image pickup sensor oftwo-dimensional pixel arrangement consisting of horizontal and verticallines. FIGS. 4(a), 4(b), 5(a) and 5(b) are schematic diagrams forillustrating a noise removal method according to an embodiment 2 of thepresent invention. FIG. 4(a) illustrates signal levels corresponding torespective pixels of a vertical line VL1 of an image pickup sensorhaving two-dimensional pixel arrangement, FIG. 4(b) schematically showspixel arrangement S1 of the image pickup sensor, FIG. 5(a) illustratessignal levels corresponding to pixels of a horizontal line HL1 of theimage pickup sensor having the two-dimensional pixel arrangement, andFIG. 5(b) schematically shows pixel arrangement S1 of the image pickupsensor.

[0040]FIG. 4(a) shows zigzag noise signal levels I[−2, 0], I[−1, 0],I[0, 0], I[1, 0], I[2, 0], . . . appearing on pixels n[−2, 0], n[−1, 0],n[0, 0], n[1, 0], n[2, 0], . . . of the vertical line VL1 shown in FIG.4(b). FIG. 5(a) shows zigzag noise signal levels I[0,−2], I[0, −1], I[0,0], I[0, 1], I[0, 2], appearing on pixels n[0,−2], n[0,−1], n[0, 0],n[0, 1], n [0, 2], . . . of the horizontal line HL1. The output levelsof these noise levels are detected when the difference between thesignal levels of two pixels adjacent to each other in a noise generationdirection repetitively changes while alternately taking positive andnegative values at least three times such that the difference ispositive, negative, positive, negative, . . . , similarly to the case ofthe aforementioned embodiment 1. As described in the above embodiment 1,these types of zigzag noise signal levels are generated around a Nyquistfrequency due to the hardware structure of an A/D converter or imageprocessing, particularly pixel interpolation.

[0041] This embodiment is characterized in correcting a signal levelI[0, 0] of a specific pixel n[0, 0] of a zigzag noise signal by aweighted mean value calculated by either the following formula (F2) or(F3): $\begin{matrix}{{\left. {I\left\lbrack {0,0} \right\rbrack}\leftarrow{\sum\limits_{i = 1}^{m}\quad \frac{{2{I\left\lbrack {0,0} \right\rbrack}} + {I\left\lbrack {i,0} \right\rbrack} + {I\left\lbrack {{- i},0} \right\rbrack}}{4m}} \right.\left( {m:{{natural}\quad {number}}} \right)}} & ({F2}) \\{\left. {I\left\lbrack {0,0} \right\rbrack}\leftarrow{\sum\limits_{i = 1}^{m}\quad \frac{{2{I\left\lbrack {0,0} \right\rbrack}} + {I\left\lbrack {0,i} \right\rbrack} + {I\left\lbrack {0,{- i}} \right\rbrack}}{4m}} \right.\left( {m:{{natural}\quad {number}}} \right)} & ({F3})\end{matrix}$

[0042] The above formulas (F2) and (F3) are correction formulas fornoise signals on the vertical and horizontal lines VL1 and HL1respectively.

[0043]FIG. 6 is a flow chart showing exemplary noise removal accordingto the embodiment 2. This example is described with reference to a caseof detecting and correcting five zigzag noise signals on each of thevertical line VL1 and the horizontal line HL1.

[0044] Image signals picked up by an image pickup sensor are A/Dconverted and thereafter input in an image processing part (ST10), to besubjected to image processing such as image interpolation, color spaceconversion, gamma correction and edge enhancement in the imageprocessing part (ST11). Then, the process shifts to a step ST12, forsuccessively storing the image signals subjected to image processing infive buffer memories (not shown) and performing the aforementionedzigzag noise signal level detection on the stored image signals everyvertical line. In other words, detection is executed on a noise signalsatisfying any of the following conditional expressions (C-1) to (C-4)among signals corresponding to the pixels n[−2, 0], n[−1, 0], n[0, 0],n[1, 0], n[2, 0], . . . of the vertical line VL1 shown in FIG. 4(b):

I[−2, 0]>I[−1, 0], and I[−1, 0]<I[0, 0], and I[0, 0]>I[1, 0]  (C-1)

I[2, 0]>I[1, 0], and I[1, 0]<I[0, 0], and I[0, 0]>I[−1, 0]  (C-2)

I[−2, 0]<I[−1, 0], and I[−1, 0]>I[0, 0], and I[0, 0]<I[1, 0]  (C-3)

I[2, 0]<I[1, 0], and I[1, 0]>I[0, 0], and I[0, 0]<I[−1, 0]  (C-4)

[0045] Whether or not such a noise signal is detected is determined at astep S13, so that the process shifts to a step ST15 described later whenthe noise signal is not detected while correction of I[0, 0]←(I[−1,0]+2×n[0, 0]+n[1, 0])/4 is executed on the noise signal level I[0, 0] ofthe specific pixel n[0, 0] on the basis of the above formula (F2) at astep ST14 and the process thereafter shifts to the step ST15 when thenoise signal is detected.

[0046] At the step ST15, a zigzag noise signal level on the horizontalline HL1 is detected. In other words, detection is executed on a noisesignal satisfying any of the following conditional expressions (D-1) to(D-4) among signals corresponding to the pixels n[0,−2], n[0,−1], n[0,0], n[0, 1], n[0, 2], . . . of the horizontal line HL1 shown in FIG.5(b):

I[−2, 0]>I[−1, 0], and I[−1, 0]<I[0, 0], and I[0, 0]>I[1, 0]  (D-1)

I[2, 0]>I[1, 0], and I[1, 0]<I[0, 0], and I[0, 0]>I[−1, 0]  (D-2)

I[−2, 0]<I[−1, 0], and I[−1, 0]>I[0, 0], and I[0, 0]<I[1, 0]  (D-3)

I[2, 0]<I[1, 0], and I[1, 0]>I[0, 0], and I[0, 0]<I[−1, 0]  (D-4)

[0047] Whether or not such a noise signal is detected is determined at astep ST16, and the image signals are output at a step ST18 and theaforementioned noise removal processing is ended if the noise signal isnot detected, while correction of I[0, 0]←(I[0, −1]+2×n[0, 0]+I[0, 1])/4is executed on the noise signal level I[0, 0] of the specific pixel n[0,0] on the basis of the above formula (F3) at a step ST17 and thereafterthe image signals from which the noise signal is removed are output(ST18) when the noise signal is detected. The noise removal is thusended.

[0048] While the noise signal is detected and removed through theaforementioned steps ST12 to ST17 after the image processing at theaforementioned step ST11 in the above noise removal, the aforementionedsteps ST12 to ST17 may alternatively be assembled into the imageprocessing at the aforementioned step ST11. For example, theaforementioned steps ST12 to ST17 may be executed after executing pixelinterpolation at the aforementioned step ST11, for thereafter executingother image processing such as color space conversion and gammacorrection.

[0049] Thus, according to the embodiment 2, only a zigzag noise signalmixed into image signals output from an image pickup sensor oftwo-dimensional image arrangement can be extracted and individuallyremoved, whereby it is possible to remove a noise signal around aNyquist frequency resulting from the hardware structure of an A/Dconverter or from pixel interpolation without reducing the overallpicture quality.

[0050] Embodiment 3.

[0051] With reference to an embodiment 3 of the present invention,processing performed when a specific pixel n[0, 0] appears on anintersection of horizontal and vertical lines between a zigzag noisesignal appearing on a horizontal line and a zigzag noise signalappearing on a vertical line in two-dimensional pixel arrangement of animage pickup sensor is described. FIGS. 7(a) and 7(b) are schematicdiagrams for illustrating a noise removal method according to theembodiment 3 of the present invention. FIG. 7(a) illustrates signallevels corresponding to pixels of a vertical line VL2 of an image pickupsensor having two-dimensional pixel arrangement, FIG. 7(b) illustratessignal levels corresponding to pixels of a horizontal line HL2 thereof,and FIG. 8 schematically shows pixel arrangement S2 of the image pickupsensor.

[0052] As shown in FIGS. 7(a) and 7(b), pixels n[−2, 0], n[−1, 0], n[0,0], n[1, 0], n[2, 0], . . . of zigzag noise signal levels appearing onthe horizontal line HL2 in the pixel arrangement S2 and pixels n[0,−2],n[0,−1], n[0, 0], n[0, 1], n[0, 2], . . corresponding to zigzag noisesignals appearing on the vertical line VL2 intersect with each other,and a specific pixel n[0, 0] is arranged on the intersection. Theembodiment 3 is characterized in correcting an output level I[0, 0] of anoise signal in the specific pixel n[0, 0] with a weighted mean valuecalculated in the following formula (F4) in such a case: $\begin{matrix}{\left. {I\left\lbrack {0,0} \right\rbrack}\leftarrow{\sum\limits_{i = 1}^{m}\quad \frac{{4{I\left\lbrack {0,0} \right\rbrack}} + {I\left\lbrack {0,i} \right\rbrack} + {I\left\lbrack {0,{- i}} \right\rbrack} + {I\left\lbrack {i,0} \right\rbrack} + {I\left\lbrack {{- i},0} \right\rbrack}}{8m}} \right.\left( {m:{{natural}\quad {number}}} \right)} & ({F4})\end{matrix}$

[0053]FIG. 9 is a flow chart showing exemplary noise removal accordingto the embodiment 3. This example is described with reference to a caseof detecting and correcting five zigzag noise signal levels in each ofthe vertical line VL2 and the horizontal line HL2 similarly to theaforementioned embodiment 2.

[0054] Image signals picked up by an image pickup sensor are A/Dconverted and thereafter input in an image processing part (ST10) andsubjected to the aforementioned image processing (ST11), so that theimage signals subjected to image processing are successively stored infive line buffer memories (not shown) at a step S20 and theaforementioned zigzag noise signal level detection is performed on thestored image signals. In other words, a noise signal satisfying any ofthe aforementioned conditional expressions (C-1) to (C-4) and (D-1) to(D-1) is detected.

[0055] Then, whether or not zigzag noise signal levels are detected onboth of the vertical line VL2 and the horizontal line HL2, i.e., whetheror not both of a noise signal (zigzag noise signal appearing in thevertical direction) satisfying any of the above conditional expressions(C-1) to (C-4) and a noise signal (zigzag noise signal appearing in thehorizontal direction) satisfying any of the conditional expressions(D-1) to (D-4) are detected is determined at a step ST22. The processshifts to a step ST23 when it is determined that zigzag noise signalsare detected on both of the vertical line VL2 and the horizontal lineHL2, for determining whether or not both specific pixels n[0, 0] matchwith each other. When the noted pixels n[0, 0] of both noise signalsmatch with each other, the process shifts to a step ST24 for executingcorrection of I[0, 0]←(I[−1, 0]+I[0, −1]+4×I[0, 0]+I[0, 1]+I[1, 0])/8 onthe basis of the above formula (F4) and outputting the image signals ata step ST30, so that the noise removal is thereafter ended.

[0056] When it is determined that no zigzag noise signals are detectedon both of the vertical line VL2 and the horizontal line HL2 in thedetermination block of the aforementioned step ST22, on the other hand,the process shifts to those following a step ST25. Whether or not azigzag noise signal is detected only on the vertical line VL2 isdetermined at the step ST25, and whether or not a zigzag noise signal isdetected only on the horizontal line HL2 is determined at a step ST27.When the zigzag noise signal is detected only on the vertical line VL2,correction of I[0, 0]←(I[−1, 0]+2×n[0, 0]+n[1, 0])/4 is executed on thenoise signal level I[0, 0] of the specific pixel n[0, 0] on the basis ofthe above numerical formula (F2) (ST26). When the zigzag noise signal isdetected only on the horizontal line HL2, on the other hand, correctionof I[0, 0]←(I[0, −1]+2×n[0, 0]+I[0, 1])/4 is executed on the noisesignal level I[0, 0] of the specific pixel n[0, 0] (ST28).

[0057] When it is determined that the specific pixels n[0, 0] mismatchwith each other at the aforementioned step ST23, the processing of theaforementioned steps ST26 and ST28 is executed on the vertical line VL2and the horizontal line HL2 respectively at a step ST29. Then, the noiseremoval is ended.

[0058] Thus, according to the embodiment 3, it is possible to performcorrection on zigzag noise signals appearing on both of vertical andhorizontal lines inclusive of a case where specific pixels thereof matchwith each other. Thus, an image of better quality can be provided.

[0059] While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

What is claimed is:
 1. A method of removing noise from an image signal,comprising steps of: (a) detecting such a zigzag signal that thedifference between the output levels of two adjacent pixels alternatelytakes positive and negative values at least three times in pixelarrangement along a prescribed direction; and (b) noting a specificpixel among a plurality of pixels corresponding to said zigzag signaland calculating a mean value of the output levels of signalscorresponding to said plurality of pixels with reference to saidspecific pixel for correcting the output level of a signal correspondingto said specific pixel to said mean value.
 2. The method of removingnoise from an image signal according to claim 1, wherein said zigzagsignal is generated as a straight line in said image signal in said step(a), said method calculating a weighted mean of the output level of saidzigzag signal with reference to said specific pixel for correcting theoutput level of said signal corresponding to said specific pixel to saidweighted mean in said step (b).
 3. The method of removing noise from animage signal according to claim 1, wherein said image signal is atwo-dimensional image signal formed by horizontal and vertical lines,and said zigzag signal is intersectionally generated in two directionsof said horizontal and vertical lines in said image signal in said step(a), said method calculating a weighted mean of the output level of saidzigzag signal generated along said two directions with reference to saidspecific pixel at the intersection of said horizontal and vertical linesfor correcting the output level of said signal corresponding to saidspecific pixel to said weighted mean in said step (b).